[IA64] sn: Fix zeroing of PDAs

The code uses a the following to zero out a PDA:

	memset(pda, 0, sizeof(pda));

But sizeof(pda) will return the size of a pointer rather than the size
of the structure pointed to. This triggers the following warning from
GCC:

	arch/ia64/sn/kernel/setup.c:582:23: warning: argument to 'sizeof' in 'memset' call is the same pointer type 'struct pda_s *' as the destination; expected 'struct pda_s' or an explicit length [-Wsizeof-pointer-memaccess]
	  memset(pda, 0, sizeof(pda));
	                       ^

Fix this by passing in the size of the structure using sizeof(*pda)
instead.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>